Orthogonal Frequency-Division Multiple Access (OFDMA) is a multiple access technique where access is shared among the different users in the frequency domain, by subdividing the system bandwidth into a number of subcarriers enjoying orthogonal access. Resources can also be assigned in the time domain on a per-user basis. The available subcarriers can thus be scheduled to different users and signals in a very flexible way, by employing both frequency and time domains. A number of communication systems employ OFDMA for both wireless and wired (fixed) networks, such as Long-Term Evolution (LTE) or Asymmetric Digital Subscriber Line (ADSL), among others.
Long-Term Evolution (LTE) is the next step in cellular Third-Generation (3G) systems, which represents basically an evolution of present mobile communications standards, such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM) [1]. It is a Third Generation Partnership Project (3GPP) standard that provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink. It uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations, employing OFDMA as multiple access technique. LTE is also expected to improve spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth.
LTE-Advanced (LTE-A), an evolution of LTE, is being standardized in LTE Release 10 and beyond. It is aimed at fulfilling International Mobile Telecommunications (IMT)-Advanced requirements, whose capabilities go beyond those of IMT-2000 and include enhanced peak data rates to support advanced services and applications (100 Mbps for high mobility, and 1 Gbps for low mobility).
One of the growing fields of interest in LTE is so-called Radio Access Network (RAN) virtualization, whereby improved RAN architectures are being explored for higher flexibility of operation. Examples of RAN virtualization are so-called Cloud-RAN (where baseband processing of multiple cells is centralized and radio access is provided through a set of remote radio heads), and network-in-a-cell (the opposite case, where the whole radio access and core network are integrated in one site). All these examples can leverage on generic hardware for processing purposes and different software modules for the different network nodes.
All the above solutions share the property of keeping the fundamental properties of the original radio access technology unchanged prior to virtualization, such as carrier frequency, access technique, physical layer or frequency of operation.
At the same time, there exist situations where two or more RATs must coexist for deploying basic services. This happens in LTE, where data-only services are available and legacy circuit-switched (CS) connections must be served through appropriate redirection/handover to a different RAT (so-called Circuit Switched Fallback, or CSFB) [2]. In these cases redirection to a legacy RAT implies losing the previous LTE connection as a different frequency band is involved, unless terminals are equipped with dual radio frequency front-ends.
RAT coexistence is sometimes approached through exploiting gaps in the transmission of a given RAT (e.g. blank subframes in LTE), where no physical signal is present, and filling them with transmissions of a second RAT (e.g. WiMAX) that fit the timing of the first RAT as disclosed in U.S. Pat. No. 8,547,989 “Methods and Systems for LTE-WiMAX Coexistence”. Such an approach avoids interference between RATs when they use the same frequencies or frequencies so close to each other that significant interference would appear.
Terminals capable of transmitting and receiving simultaneously in two RATs are rarely seen in cellular networks due to increased cost and power consumption. Instead, multi-RAT terminals are often limited to transmit and receive using only one RAT at a time, therefore precluding simultaneous operation of multiple RATs. Simultaneous operation in low-power unlicensed bands is usually allowed (e.g. WiFi and Bluetooth) but not in licensed bands reserved for cellular use where a given quality of service must be guaranteed.
If not implementing dual radios, terminals must switch between RATs when necessary. As an example, CSFB from LTE to a 3G/2G RAT poses significant issues in order to take care of an incoming/outgoing voice call. As the two RATs involve different frequencies, the user terminal must spend a significant time searching for the other RAT in order to acquire system information prior to connecting to the other cell. Furthermore, the switching from one frequency band to the other must be done in a very short period of time. After completion of the voice call the user must also reselect to any LTE cell as soon as possible, and this mechanism is not always efficient and usually suffers from large return times to LTE.
The fragmentation of LTE spectrum has motivated the introduction of Carrier Aggregation capabilities in some real networks. This fact, together with the necessary support of other RATs and frequency bands, complicates the RF structure of the terminals as a high number of transceivers must be integrated into a single device. Re-farming of legacy spectrum for LTE purposes undergoes a slow and difficult process, and in the meantime devices must cope with a high number of frequency bands which must be prioritized due to space and power consumption restrictions.
On the other hand, coexistence of LTE and other data-capable RATs (such as UMTS/HSPA) is not usually exploited in a coordinated way. As an example, it is difficult to exploit the differences in cell load for inter-RAT load balancing purposes, unless some proprietary interaction between the schedulers of both cells is implemented. Other solutions for seamless inter-RAT operation would therefore be desirable.
Some advanced techniques such as inter-RAT Carrier Aggregation (e.g. combining HSPA and LTE) will also require simultaneous operation in two or more RATs. In these cases the traditional solution comprises dual radio transceivers with the subsequent increase in cost, volume and power consumption.
Some hybrid solutions do exist for a combined Orthogonal Frequency Division Multiple Access (OFDMA) and Code Division Multiple Access (CDMA), such as the one described in U.S. Pat. No. 8,427,936 “Multiple-Access Hybrid OFDM-CDMA System”, where CDMA multiple access is performed over the frequency domain of an OFDM system. These proposals only address the combined OFDM and CDMA operations for a hybrid multiple access system, but no attention is given to seamless coexistence and inter-operation of several radio access technologies using the same radio frequency front-end.
The solution in U.S. Pat. No. 8,547,989 requires a switching time between RATs to avoid interference and thus precludes simultaneous operation of the two RATs. In addition, two separate radio transceivers are still required at the device because of the fundamentally different bandwidths, sampling frequencies, guard bands, or emission limits that characterize the signals in the two RATs. Devices would therefore need to incorporate two radio frequency front-ends with the same increase in cost, volume and power consumption than dual radio transceivers.
More efficient ways for dealing with multiple RATs in the same frequency band are therefore desirable in order to reduce the complexity of multi-RAT devices.